1. Field of the Invention
The present invention relates broadly to bonding one article to another. More particularly, the invention relates to bonding electrical leads to substrates. For example, an integrated circuit package usually has leads which extend from such package to permit it to be assembled into a larger circuit system. Inside the package, however, the leads are bonded to a microcircuit. A known package structure uses a small ceramic or glass substrate as a base element to which the leads and one or more semiconductor chips are bonded. A metal pattern on the substrate is coupled to contacts on the chips and also provides bond sites for the inner ends of the leads. While the present invention ideally applies to the described circuit package, it is not limited to such an area of technology.
2. Description of the Prior Art
A common bonding technique for attaching leads to a substrate is "thermocompression bonding." According to this technique a metal-to-metal bond is established between a metallized bond site on the substrate and a lead through the combination of thermal and compressive energy. The energy is applied to the lead through a thermode and is directed toward the corresponding bond site of the substrate to join the metal at the interface between the lead and the substrate at temperatures below the melting point of the interface metal. Typical structures for such substrate-lead bonds involve glass or ceramic substrates having thin film metal patterns which are in the order of 20,000 angstroms thick. These metal patterns form the bond sites and the interconnecting conductors to circuit chips mounted to the substrate. The leads to be bonded are typically 0.020 by 0.010 inch in cross section. As a bonding force is applied to the leads by the thermode, the lead material deforms and flattens, and the bond between the leads and the corresponding bond sites is formed.
In bonds which have been formed in accordance with the outlined technique, bond failures tend to occur which are characterized by the separation of bonded leads from their bond sites and by the occurrence of a break in the substrate material itself rather than at the interface between the lead and the bond. The failure mechanism is consequently commonly referred to as substrate pullout or substrate cavitation.
Various studies have been undertaken to overcome the problem of substrate cavitation. One of these studies resulted in the invention disclosed in U.S. Pat. No. 3,834,604 to Fendley et al. which is assigned to the assignee of the present invention. According to the Fendley et al. disclosure, blanked-out leads which typically have been employed by the industry are modified to the extent that preferably two or more interposed and specially contoured compliant contacts are formed on each of the leads to be bonded to the substrate. The preshaping of the contacts into pyramidal shapes of essentially triangular cross-section provides compliancy and further produces redundant bonds to insure against total bond failures.
While the Fendley et al. procedure tends to minimize or to overcome the problem of failures by cavitation between leads and ceramic or glass substrates, special equipment for preshaping the compliant bond pads onto the ends of the leads is required. The additional equipment expense adds to the manufacturing cost of circuit packages, tending to offset gains from increases in yields because of decreased bond failures.
It is, therefore, desirable to minimize bond failures by cavitation and, at the same time, use leads having ends which require no special preparation for bonding operations.